1. Field of the Invention
The present invention relates to a sensor control apparatus, a sensor control system, and a sensor control method adapted to calculate a concentration correspondence value that represents the concentration of a specific gas contained in a gas to be detected (hereinafter referred to as an “object gas”).
2. Description of the Related Art
Conventionally, a gas sensor has been utilized which detects the concentration of a specific gas contained in an object gas such as exhaust gas. For example, an NOX sensor, which detects nitrogen oxides (hereinafter referred to as “NOX”) as a specific gas, includes a detection element which has an oxygen concentration detection cell, a first oxygen pump cell, and a second oxygen pump cell, each composed of an oxygen-ion conductive solid electrolyte layer and porous electrodes formed thereon. The first oxygen pump cell pumps oxygen out of a first measurement chamber into which an object gas is introduced, or pumps oxygen into the first measurement chamber. The second oxygen pump cell pumps oxygen out of a second measurement chamber that is in communication with the first measurement chamber.
A control apparatus for the NOX sensor supplies current to the first oxygen pump cell so as to pump oxygen out of the first measurement chamber or pump oxygen into the first measurement chamber such that the oxygen concentration detection cell facing the first measurement chamber outputs a constant voltage. Thus, the oxygen concentration of the object gas within the first measurement chamber is controlled to a constant level. Also, the control apparatus applies a constant voltage between the electrodes of the second oxygen pump cell so as to pump oxygen out of the gas introduced from the first measurement chamber into the second measurement chamber (the gas whose oxygen concentration has been adjusted by the first oxygen pump cell). As a result of applying the constant voltage, NOX within the gas is decomposed, and oxygen ions originating from the NOX flow through the second oxygen pump cell. The control apparatus detects the concentration of NOX within the object gas on the basis of the current flowing through the second oxygen pump cell.
In the case where the concentration of NOX contained in, for example, exhaust gas discharged from an internal combustion engine is detected by use of an NOX sensor, the gas present in the second measurement chamber approaches a lean state close to that of the atmosphere, with the passage of time from when the previous operation of the internal combustion engine has been stopped to restart thereof. In view of such a phenomenon, some control apparatuses are configured to perform, at the time of startup of the internal combustion engine, preliminary control so as to temporarily and rapidly pump oxygen out of the second measurement chamber. Since the preliminary control can lower the oxygen concentration within the second measurement chamber to a predetermined low oxygen concentration, at which the concentration of NOX can be detected and at an earlier timing as compared with the case where the preliminary control is not performed, the control apparatus can start detecting the NOX concentration at an earlier timing as compared to the case where preliminary control is not performed.
Also, the oxygen concentration within the second measurement chamber at the time when the preliminary control ends is lower than the predetermined low oxygen concentration at which the concentration of NOX can be detected. At the beginning of drive control for driving the detection element subsequent to preliminary control, oxygen is pumped back into the second measurement chamber such that the voltage applied to the second oxygen pump cell becomes the above-mentioned constant voltage. After the oxygen concentration within the second measurement chamber assumes the predetermined low oxygen concentration and the output of the detection element becomes stable, detection of the NOX concentration is performed.
Incidentally, it has been known that, when the voltage applied to the second oxygen pump cell is equal to or higher than a predetermined value, dissociation of moisture (H2O) contained in the object gas occurs on an electrode of the second oxygen pump cell. It has also been known that, since oxygen ions generated as a result of dissociation of H2O flow between the electrodes of the second oxygen pump cell, the current flowing therethrough increases in accordance with the concentration of H2O contained in the object gas. In the case where a high voltage equal to or higher than the predetermined value is applied to the second oxygen pump cell during the preliminary control, even when the same detection element is used, the oxygen concentration within the second measurement chamber at the end of the preliminary control changes depending on the H2O concentration. Therefore, in a period during which oxygen is pumped back after the start of drive control, the output value of the detection element exhibits a time course change (a change with time), and the trend of the time course change (in other words, a change in the output value with time, or a curve (pattern) depicted on a graph which shows the relation between the output value of the detection element and elapse of time) changes depending on the H2O concentration.
A control apparatus which addresses the above-described problem has been known (for example, see Patent Document 1). The known control apparatus supplies current of a predetermined magnitude to the second oxygen pump cell for a certain period of time such that the amount of oxygen pumped out by the second oxygen pump cell becomes constant. When the control apparatus performs such control, the oxygen concentration within the second measurement chamber at the end of the preliminary control can be made substantially constant irrespective of the H2O concentration. Therefore, the trend of the time course change of the output value of the detection element in the period during which pumping back of oxygen is performed after the start of the drive control becomes constant irrespective of the H2O concentration, which enables the control apparatus to stably measure the NOX concentration.    [Patent Document 1] Japanese Patent Application Laid-Open (kokai) No. 2010-156676
3. Problems to be Solved by the Invention
However, even when the time course change exhibits a substantially constant trend at the beginning of the period during which oxygen is pumped back after the start of the drive control, the timing at which the output value of the detection element exhibits the trend of the time course change may deviate from an ideal timing because of individual differences among detection elements (variation among individual detection elements). The conventional control apparatus does not detect the concentration of NOX at the beginning of the drive control, waits for elapse of a predetermined time, and starts the detection of the NOX concentration after the output of the detection element becomes stable. Therefore, the above-mentioned deviation of the timing at which the output value of the detection element exhibits the trend of the time course change in the period during which oxygen is pumped back has not been taken into consideration.